The present invention is directed to an allogenic implant and, more particularly, to an allogenic intervertebral implant.
A number of medical conditions such as compression of spinal cord nerve roots, degenerative disc disease, and spondylolisthesis can cause severe low back pain. Intervertebral fusion is a surgical method of alleviating low back pain. In posterior lumbar interbody fusion (xe2x80x9cPLIFxe2x80x9d), two adjacent vertebral bodies are fused together by removing the affected disc and inserting an implant that would allow for bone to grow between the two vertebral bodies to bridge the gap left by the disc removal.
A number of different implants and implant materials have been used in PLIF with varying success. Current implants used for PLIF include threaded titanium cages and allografts. Threaded titanium cages suffer from the disadvantage of requiring drilling and tapping of the vertebral end plates for insertion. In addition, the incidence of subsidence in long term use is not known. Due to MRI incompatibility of titanium, determining fusion is problematic. Finally, restoration of lordosis, i.e., the natural curvature of the lumbar spine is very difficult when a cylindrical titanium cage is used.
Allografts are sections of bone taken from a long bone of a donor. A cross section of the bone is taken and processed using known techniques to preserve the allograft until implantation and reduce the risk of an adverse immunological response when implanted. For example, U.S. Pat. No. 4,678,470 discloses a method for processing a bone grafting material which uses glutaraldehyde tanning to produce a non-antigenic, biocompatible material. Allografts have mechanical properties which are similar to the mechanical properties of vertebrae even after processing. This prevents stress shielding that occurs with metallic implants. They are also MRI compatible so that fusion can be more accurately ascertained and promote the formation of bone, i.e., osteo-conductive. Although the osteoconductive nature of the allograft provides a biological interlocking between the allograft and the vertebrae for long term mechanical strength, initial and short term mechanical strength of the interface between the allograft and the vertebrae are lacking as evidenced by the possibility of the allograft being expelled after implantation.
Currently commercially available allografts are simply sections of bone not specifically designed for use in PLIF. As a result, the fusion of the vertebral bodies does not occur in optimal anatomical position. A surgeon may do some minimal intraoperative shaping and sizing to customize the allograft for the patient""s spinal anatomy. However, significant shaping and sizing of the allograft is not possible due to the nature of the allograft. Even if extensive shaping and sizing were possible, a surgeon""s ability to manually shape and size the allograft to the desired dimensions is severely limited.
Most PLIF implants, whether threaded cages or allograft, are available in different sizes and have widths that vary with the implant height. For example, the width of a cylindrical cages will be substantially equivalent to the height. Although larger heights may be clinically indicated, wider implants are generally not desirable since increased width requires removal of more of the facet, which can lead to decreases stability, and more retraction of nerve roots, which can lead to temporary or permanent nerve damage.
As the discussion above illustrates, there is a need for an improved implant for fusing vertebrae.
The present invention relates to an allogenic intervertebral implant for use when surgical fusion of vertebral bodies is indicated. The implant comprises a piece of allogenic bone conforming in size and shape with a portion of an end plates of the vertebrae and has a wedge-shaped profile with a plurality of teeth located on top and bottom surfaces. The top and bottom surfaces can be flat planar surfaces or curved surfaces to mimic the topography of the end plates. The implant has a channel on at least one side for receiving a surgical tool. This channel runs in the anterior direction to accommodate a variety of surgical approaches. A threaded hole on the anterior, posterior, posterior-lateral, or lateral side can be provided for receiving a threaded arm of an insertion tool.
In another embodiment, the implant has an interior space for receiving an osteoconductive material to promote the formation of new bone.
In another embodiment, the implant is made of a plurality of interconnecting sections with mating sections. Preferably, the implant is made in two halves: a top portion having a top connecting surface and a bottom portion having a bottom connecting surface. The top connecting surface mates with the bottom connecting surface when the top and bottom portions are joined. The top and bottom portions have holes that align for receiving a pin to secure the top and bottom portions together. The pin can be made of allogenic bone.
In a different embodiment, the medial side of the implant has a scalloped edge such that when a first implant is implanted with a second implant with the medial sides facing each other, the scalloped edges define a cylindrical space.
The present invention also relates to a discrete spacer used in conjunction with any of the other embodiments of the implant. The spacer comprises a piece of allogenic bone conforming in size and shape with a portion of an end plates of the vertebrae and has a wedge-shaped profile with substantially smooth top and bottom surfaces. The intersecting regions between the top and bottom surfaces and at least one of the lateral sides and the intersecting regions between the anterior and posterior sides and the same lateral side are curved surfaces to facilitate implantation of the spacer. Thus, the spacer can be implanted through an opening on one side of the spinal canal and moved with a surgical instrument to the contralateral side.